1. Field of the Invention
The invention relates to the field of nondestructive analysis of phosphate coated surfaces and in particular to the nondestructive determination of the thickness and composition of the phosphate layer of an automotive surface.
2. Description of the Prior Art
In recent years the cathodic electro-coating of paint primers onto phosphated steel has become increasingly important. This is particularly true in the automotive industry where the process is used as a precursor to undercoating and topcoating with acrylic base paints. In many instances the performance of the total corrosion resistance thus produced has been remarkably good, though particular care is required in selecting the correct phosphate system and application process. A particular example of such a phosphate coating for use in automotive paints is described in Richardson et al., "The Influence of Zinc Phosphate Crystalline Morphology on the Corrosion Resistance of Electro-Painted Steel", Transactions of the Institute of Metal Finishing, Volume 61, page 155 (1983).
Prior art methods for providing quality control of a phosphated steel surface comprised the steps of removing small chips of the phosphate coated surface from the automobile on the production line, transporting the chips to a laboratory, sometimes distantly separated from the assembly line, and then separating the phosphate layer from the substrate. The examination of the depth and composition of the layer was then made under a microscope with results reported back to the production line.
Clearly, with such prior art methods, even when the laboratory is located on site and at the immediate disposal of the quality control personnel on the manufacturing line, a substantial delay exists between the time when the sample is taken from the coated surface and when the results are reported. The cost and delay not only makes quality checking of each coated surface of an automobile impractical, but it tends to cause practical quality control by sampling techniques to be at best an after-the-fact determination. Depending upon the circumstances of manufacture, a corrective response cannot be made, (if one is indicated) on items being installed within assembled products. Correction can only be made to subsequently finished components. In the meantime, process variables may have changed and subsequent finished components may or may not continue to have the same defect or be defective to the same extent as the previously sampled product.
Therefore, what is needed is an on-line real-time nondestructive means for determining the thickness and composition of the phosphate layer on a steel surface.